The theoretical efficiency of a solar cell is high, but the actual power generation efficiency is not the case. There are many factors affecting the cell efficiency, such as series and parallel resistances of the solar cell itself, the shielding of sunlight by the solar cell electrodes, or the loss caused by not effectively capturing the reflected light, etc.
The recombination of electrons and holes inside solar cell is another important factor affecting the cell efficiency. The surface or interface of the semiconductor material of solar cell will produce more lattice defect density, such as dangling bonds. The dangling bonds act as defect traps and therefore are sites of recombination of electron-hole pairs.
Passivation is a commonly used method to minimize the electron-hole pairs recombination at the surface of a solar cell. The two well-known surface passivation methods are chemical passivation and field-effect passivation.
The chemical passivation includes depositing a dielectric material on the surface of a n-type or p-type semiconductor with a thermal oxidation process. The thermal oxidation process provides a good interface passivation quality through chemical bonding between the dielectric layer and the semiconductor, and results in defect density reduction at the interface of the semiconductor/dielectric layer, thereby lowering the recombination probability of the electrons and holes on a semiconductor surface.
The field-effect passivation approach adopts dielectric material to inhibit the recombination of minority carriers. The surface passivation behavior of a dielectric layer depends crucially on the fixed charges polarity in the dielectric layer and the doping concentration of the semiconductor. For example, for n-type semiconductor, silicon nitride having positive charges induces majority carriers (electrons) accumulation at the dielectric/semiconductor interface, causing the energy bands to bend downward. The minority carriers (holes) are shielded from the crystalline silicon solar cell surface.
From the view of structure, the passivation layer formed by both the chemical passivation and the field-effect passivation methods directly contacts the semiconductor photoelectric conversion layer of a solar cell.